The MLB's letter said Zobrist will be subject to discipline, including a fine, if he continued to wear non-compliant shoes.

Published at 6:58 PM CDT on May 14, 2018

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In this April 12, 2018, file photo, Chicago Cubs second baseman Ben Zobrist, left, misses the tag on Pittsburgh Pirates Corey Dickerson, right, at second base during the ninth inning of a baseball game in Chicago. Major League Baseball has warned Zobrist of the against wearing black cleats. Zobrist posted a letter from the league office on Instagram saying the cleats he wore May 2 against Colorado violated the collective bargaining agreement.

Ben Zobrist was joined by Chicago Cubs teammates Kyle Schwarber and Steve Cishek in wearing black spikes on Monday despite a warning from Major League Baseball that he was in violation of the sport's uniform regulations.

Zobrist posted a letter from MLB on Instagram on Saturday that cited him for violating regulations requiring at least 51 percent of the exterior of a player's shoes be the club's designated primary shoe color.

For the Cubs, that color is blue.

“I have worn black spikes exclusively at Wrigley Field for day games to pay homage to the history of our great game,” he said. “When I was a kid, I was inspired by highlights of the greats such as Ernie Banks and Stan Musial…and was captured by the old uniforms and all black cleats with flaps.”

Zobrist also included a picture of the special “PF Flyer” cleats that he had made to honor the baseball film “The Sandlot,” and expressed annoyance that MLB was only choosing to crack down on his shoe choices now.

The MLB's letter said Zobrist will be subject to discipline, including a fine, if he continued to wear non-compliant shoes.